These studies of the basic mechanisms of the immune response to virally determined tumor associated antigens are designed to evaluate the subpopulations of lymphocytes involved in the recognition, triggering and effector phases of the immune response. Thymectomized irradiated animals repopulated with thymus and bone marrow or bone marrow alone will be immunized against polyoma and SV-40 induced cell surface antigens. The antibody response will be followed by immunofluorescence and complement dependent cytotoxicity. Neonatal hosts will be inoculated with graded doses of irradiated polyoma tumor cells and challenged as adults with viable cells to determine if tolerance either partial or complete plays a role in tumor development. The effector phase of the immune response to polyoma, SV-40 and Murine sarcoma virus induced tumor cells will be analyzed to determine the antibody class produced, the active subpopulations of effector cells, and the combined activity of fractionated antibody and fractionated lymphoid cells. In vitro functional assays will involve cytotoxicity by microcytotoxicity assays and Cr51 release tests. The lymphoid cells will be analyzed for their surface receptors for antigen antibody complexes.